1. Field of the Invention
This invention relates to the treatment of metals which have been precipitated from aqueous waste solutions to prevent the redissolution and/or reoxidation of the precipitated metal.
2. Description of the Prior Art
A commonly used method of removing metal ions from industrial effluents to allow compliance with EPA and local discharge requirements is chemical reduction. Sodium borohydride sodium dithionite (hydrosulfite), and hydrazine are typical reducing agents for this application. In a waste treatment scheme, they are added to the effluent to reduce the contained metal ions to an insoluble precipitate (which for the most part is the zero valent metal). The precipitate is then allowed to settle in a clarifier. After an appropriate residence time, the supernatant liquid in the clarifier is decanted, and either discharged directly or passed through a filter prior to discharge. Instances occur, however, when the precipitated metals redissolve by reoxidation before the solid/liquid separation step can be conducted. Such instances are particularly common in dilute solutions when the precipitate often consists of fine particles, and when metal complexing agents, such as ammonia, EDTA and gluconate are present in the solution. Redissolution can occur to such an extent that metals levels in the solution increase above discharge limits. The solution must then be treated a second time.
Alternate treatment methods for these waste streams containing complexed metal ions are known in the literature. The aforementioned complexing agents can be "tied up" by addition of metal ions that form stronger complexes with these agents than do the metal ions to be removed. Ferrous sulfate and lime (Ca(OH).sub.2) are the most commonly used additives for this purpose. However, their use results in formation of very large sludge volumes that can make succeeding steps (clarification, filtration) difficult and time-consuming. Also, cost of disposal of the sludge is thereby increased. Destruction or removal of complexing agents so that they cannot recombine with the precipitated reduced metal have been described. Destruction of free EDTA must be done under highly acid conditions using strong oxidizing agents. When the EDTA is present in a stable complex ion with a metal such as copper [Cu(EDTA).sup.2- ], the destruction is more difficult, if not impossible. Removal of ammonia can be accomplished by heating with air-sparging or by acid addition to convert NH.sub.3 to non-complexing NH.sub.4.sup.+. The former is an energy-intensive process, while the latter requires that low pH conditions be maintained throughout the process to prevent reformation of NH.sub.3. Removal of dissolved oxygen can help to inhibit redissolution of precipitated reduced metals. The most frequently used oxygen scavenger in the industry is hydrazine. However, use of hydrazine is undesirable because of its toxic nature, and should be avoided if possible.
The present invention employs bicyclic, aromatic, organonitrogen compounds such as benzotriazole, benzothiazole and benzimidazole compounds to prevent redissolution and/or reoxidation of metals (e.g. copper, nickel, cobalt) which have been precipitated from aqueous waste solutions by chemical reduction. Benzotriazoles are known to prevent corrosion of copper surfaces which are immersed in aqueous solutions. It is believed that these compounds form complexes with metal surfaces and the resulting protective layer reduces corrosion. For example, R. Walker, "Benzotriazole as a Corrosion Inhibitor for Immersed Copper," Corrosion-NACE, Vol. 29, No. 7 (1973), p. 290 discloses that when benzotriazole is added to an aqueous solution, corrosion of a copper object placed therein is reduced compared to an aqueous solution without benzotriazole. The benzotriazole forms a coating on the surface of the copper and forms a complex with the copper, thereby protecting it. Formulation of the metal/benzotriazole complex seems to be confirmed by Chadwick, D et al., Corros. Sci., 1978, 18(1), 39-51 and Ogle, I.C.G. et al., Can. Metall. Q., 1975, 14(1), 37-46. However, while benzotriazole provided corrosion inhibition in a variety of aqueous solutions, it did not inhibit corrosion in aqueous complexing and oxidizing solutions, such as those commonly found in metal-containing waste streams.
As reported in R. Walker, "Corrosion Inhibition of Copper By Tolyltriazole", Corrosion-NACE, Vol. 32, No. 8 (1976), p. 339 tolyltriazole having the formula: ##STR1## was found to be generally as good as, or better than, benzotriazole in inhibiting the corrosion of copper objects immersed in various aqueous solutions. Aqueous complexing and oxidizing solutions, however, were apparently not tested with tolyltriazole. R. Walker also discloses in Corrosion (Houston), 1975, 31(3), 97-100 the use of triazole, benzotriazole and naphthotriazole as corrosion inhibitors for copper in acidic, neutral and alkaline solutions. Naphthotriazole was found to be the most effective inhibitor.
Walker further discloses in R. Walker, "Benzotriazole a Corrosion Inhibitor for Antiques," J. Chem. Ed., Vol. 57, No. 11 (1980), p. 789 that benzotriazole can be used to protect and preserve antiques made from copper and copper alloys. Silver, gold, aluminum, brass, bronze, zinc, cadmium and steel are also cited as candidates for protection with benzotriazole.
While the aforementioned articles provide some evidence of the corrosion inhibitive properties of benzotriazoles, all of the experimental work disclosed therein was conducted on metal objects (primarily those made of copper) in the form of sheets, tubes and the like. No work was conducted on metal precipitated by chemical reduction from aqueous waste solutions and these references indicate that the benzotriazoles would not be effective in such waste solutions that contain oxidizers or complexing agents.
Japanese Patent Application No. 81202/1974 to Ochiai et al. relates to the use of benzotriazole to precipitate heavy metals from solution. The benzotriazoles are employed in amounts of 2 or more moles of benzotriazole per mole of metal and form chelates with the metal which are insoluble in the matrix liquor. The metal chelates precipitate and are recovered.
Japanese Patent Application No. 19260/1975 involves the use of a solid carrier with benzotriazole absorbed in it or a solvent which contains benzotriazole but which is not soluble in water. An aqueous solution containing heavy metals is allowed to contact the solid carrier or solvent. The metal is thereby extracted from the aqueous solution.